1. Field of the Invention
The present invention relates to a frequency measuring circuit which can measure the frequency of a signal to be measured at a high speed and with high resolution without speeding up a reference clock and a resonant pressure sensor type differential pressure/pressure transmitter using the frequency measuring circuit.
2. Description of the Related Art
FIG. 1 shows the constitution of a frequency measuring circuit used in a two-wire resonant pressure sensor type differential pressure/pressure transmitter. A resonant pressure sensor 6 outputs a signal f having frequency corresponding to a measured pressure. The signal f is inputted to a D-type flip flop 7 and is converted into a signal F which is in synchronism with a rise of a reference clock. The signal F and the reference clock are inputted to a counter 8. The counter 8 counts the reference clocks during one cycle of the signal F or a period integer times as long as the cycle so as to measure the frequency of the signal F. The frequency data are inputted to a micro computer 9 which computes the differential pressure or the pressure.
FIG. 2 shows a timing chart of the D-type flip flop 7. The reference clock is a pulse signal having a fixed frequency. The signal f is a signal having a frequency considerably lower than the frequency of the reference clock and is not in synchronism with the reference clock. The D-type flip flop 7 samples the signal f at a rise of the reference clock. Accordingly, the output F of the D-type flip flop 7 is changed in synchronism with a rise of the reference clock.
That is, the D-type flip flop 7 generates the signal F which is in synchronism with the reference clock in response to the signal f which is not in synchronism with the reference clock. Here, a gate time shown in FIG. 2 indicates a unit period in which the counter 8 counts the signal F and the reference clock.
However, the resonant pressure sensor-type differential pressure/pressure transmitter having such a constitution has a following drawback.
In the constitution shown in FIG. 1, the counter 8 is served for counting the reference clock and hence, the counter 8 cannot obtain the resolution of equal to or less than 1 cycle of the reference clock. To obtain the frequency of the output f of the resonant pressure sensor 6 at a high speed, it is necessary to shorten the gate time shown in FIG. 2. However, when the gate time is shortened, there has been a drawback that the resolution of the count value is lowered.
Although it is necessary to elevate the frequency of the reference clock to increase the resolution, this elevation of the frequency brings about the increase of power consumption. Since the two-wire differential pressure/pressure transmitter is required to exhibit the small power consumption, there has been also a drawback that the increase of the frequency of the reference clock is difficult.
Accordingly, it is an object of the present invention to provide a frequency measuring circuit which can obtain high-speed processing without elevating the frequency of a reference clock and prevents lowering of resolution and a resonant pressure sensor type differential pressure/pressure transmitter using the frequency measuring circuit.